There are materials (or textures) - either simple colours or images - that you can apply in SketchUp. Now you simply had a B&W image - but imagine tracing a shape from a real world photograph. After you successfully traced the contours of the shape (and your face is created in SketchUp), you can actually use that photo to "paint" onto this face.
Once you import an image (as image) in SU, it is a special entity. If you right click, you can "explode" it - now it becomes a rectangle with that image applied as texture on its face (and already projected, too). This way (before PushPulling), you can even apply that photo as a texture on your new shape.
Of course, the photo needs to be taken carefully; excluding any perspective distorting as much as possible (or "un-destort" it prior to importing into SU).